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Unformatted text preview: sion forces).
(C) Materials that crystallize in the ß-ZnS structure are covalent or ionic.
Each atom has four nearest neighbors in a tetrahedral coordination. The neighbors of FCC
atoms are tetrahedral atoms so this is a natural structure for equivalent ionic compounds
Under what conditions might a metallic AB compound choose the β-ZnS structure?
If the sizes of A and B atoms vary to a great extent, they would choose the B-ZnS structure.
2. Darker shaded part: A+B //Upper shaded part: B+L //Small shaded part: A+L
(A) Let a liquid with the composition indicated by the vertical line on the phase diagram
be cooled from the liquid slowly enough to preserve equilibrium. Describe the evolution
of the system as it is cooled. The system remains liquid until it reaches the temperature of the shaded B+L two-phase field.
As the temperature is cooled further, it becomes a homogenous solid solution in the B phase.
It remains homogenous until the temperature is cooled into the A+B two-phase field. Here,
small amount of B-rich beta phase precipitates out of alpha.
(B) Suppose that you are given the assignment of purifying a B-rich solution until it is...
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- Fall '12